1. Field of the Invention
This invention relates generally to heat pump systems and, more particularly, to a method for controlling supplemental electric heat during the process of defrosting the outdoor coil thereof, so as to prevent overshoot and maximize heat pump efficiency.
1. Description of the Prior Art
During operation of a conventional heat pump in the heating mode, the outdoor coil acts as an evaporator. This means that the refrigerant in the outdoor coil is at a lower temperature than the ambient air, as it must be in order to transfer heat from the ambient air to the refrigerant by way of the outdoor coil. Under commonly occurring ambient conditions, the medium--almost always air--which is in a flowing heat transfer relationship with the evaporator, has its temperature lowered below its dew point. This causes condensation to develop on the coil, resulting in the formation of frost or, if the ambient temperature is sufficiently low, ice on the outdoor coil. Because the temperature of the refrigerant in the coil is lower than that of the ambient air, ice formation may occur even at ambient temperatures above the freezing point.
The presence of ice or frost decreases the efficiency of the heat exchanger and, in turn, the efficiency and capacity of the entire system. This results in a drop of the temperature of the air supplied to the conditioned space, potentially to an uncomfortable level. It is thus desirable, if not essential, to eliminate the frost or ice from the surface of the evaporator. This is accomplished by periodically running a defrost cycle.
Again, conventionally, the outdoor coil is defrosted by reversing the refrigerant flow so that the outdoor coil functions as a condenser rather than an evaporator. The heated refrigerant gas in the outdoor coil serves to melt the ice that had been formed thereupon. When the outdoor coil serves as a condenser the indoor coil, correspondingly, serves as an evaporator, and the refrigerant removes heat from the air being blown across the indoor coil. This results in air that is at a greatly reduced temperature being returned to the conditioned space, an undesirable phenomenon known as "cold blow".
One way of dealing with the "cold blow" phenomenon is to provide electric resistance heater elements in the supply air stream, which are energized during the defrost cycle in order to provide supplemental heat to the conditioned space. If, however, insufficient heat is provided to overcome the system's cooling capacity "cold blow" still occurs. On the other hand, if too much heat is provided by the supplemental heaters, the supply air temperature will become high enough to satisfy the indoor thermostat, the defrost cycle will be incomplete, and, with ice still blocking the exchange between the outdoor coil and air, system efficiency is compromised. This condition is known as "overshoot".
In a prior art invention, U.S. Pat. No. 5,332,028 to Derrick A. Marris assigned to a common assignee, the system was provided with a plurality of units capable of providing supplemental heat, so that the amount of supplemental heat provided could be staged. The teachings of the U.S. Pat. No. 5,332,028 are herein incorporated by reference as these teachings relate to heat provision during the defrost mode. In this prior invention, provision was made for sensing the temperature of the air being supplied to the conditioned space and responsively turning on an appropriate number of supplemental heat units to achieve the correct stage of supplemental heating. In this system each defrost cycle was individually handled--that is the number of heating stages needed in the previous defrost cycle had no effect on the current defrost cycle. If, for example, all the supplemental heating units had been used during the previous defrost cycle, the system still had to add one stage at a time until sufficient, but not excessive, heat was provided.
Since defrost cycles can occur with a frequency such that little change in ambient conditions may be expected from cycle to cycle, it is desired to provide a method for having the current number of supplemental heating unit energized depend upon the number of such units needed in the most recent previous cycle and the duration of the cycle. This allows the estimated correct amount of number supplemental heating units to be activated upon entering the defrost mode, tends to eliminate both cold blow and overshoot, and increases the efficiency of the system.
Furthermore, when an overshoot condition does occur wherein the room thermostat is satisfied but the defrost is not complete, it is preferable to provide a lowered level of supplemental heat until the defrost is completed, rather than terminating the heating cycle after only a partial defrost.